Thin Solid Films 519 (2010) 1699–1704

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Spatial and temporal investigation of high power pulsed magnetron discharges byoptical 2D-imaging

B. Liebig a, N. St. J. Braithwaite b, P.J. Kelly c, J.W. Bradley a,⁎a Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool, L69 3GJ, UKb Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UKc Surface Engineering Group, Manchester Metropolitan University, Chester Street, Manchester, M1 5GD, UK

⁎ Corresponding author.E-mail address: j.w.bradley@liv.ac.uk (J.W. Bradley).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.06.055

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 February 2010Received in revised form 12 May 2010Accepted 24 June 2010Available online 1 July 2010

Keywords:High power impulse magnetron sputteringHIPIMSOptical imagingOptical emission spectroscopyPlasma diagnosticsMagnetron sputtering

Optical 2D-imaging in combination with Abel inversion was used to study the spatial and temporal evolutionof the plasma-induced emission of HIPIMS discharges. A titanium target, as well as an aluminium-doped zinctarget was sputtered in an argon atmosphere of pressures 0.53 Pa and 1.33 Pa and an average power of650 W and 400 W, respectively. The discharge was observed optically employing various wavelength filtersto investigate the development of selected species, namely argon and titanium neutrals, as well as argon andzinc neutrals and ions.The argon neutral emission did not only differ substantially from the DC case but also underwent asignificant development during the pulse ‘on’-time, showing a structure similar to an ion acoustic wavetravelling away from the target, as well as rarefaction of the working gas while sputtering with highdischarge peak currents. In addition, the intensity profile of argon and zinc ions revealed an increased widthin front of the target which is then reflected by a wider sputter distribution of the target material. Indeed, itwas found that the width of the metal neutral emission increased with increasing discharge current. Theiremission revealed two distinct maxima and the loss of intensity in between these maxima could beexplained by an increased ionisation of the sputtered metal flux.

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1. Introduction

High power impulse magnetron sputtering, HIPIMS, first intro-duced by Kouznetsov et al. [1] is now a cutting edge technology for thedeposition of thin films of engineering quality. In order to increase theflux of ionised sputtered particles towards the substrate a highinstantaneous power is delivered to the discharge while the dutycycle is reduced to prevent the magnetron source from overheating.HIPIMS discharges are generally characterised by high peak powerdensities of more than 1 kW cm−2 [2], while peak current densities inthe range of hundreds of mA cm−2 up to several A cm−2 werereported [3]. During the pulse ‘on’-time, when power is delivered tothe plasma, electron densities exceeding 1012 cm−3 have beenreported by various authors [4,5]. This leads to a high degree ofionisation of the sputtered particle flux up to 90%, depending on thetarget material and discharge parameters which has been revealed bymass spectrometry and optical emission spectroscopy [6,7]. The highflux of low energetic ionic particles penetrating the substrate canresult in a transition into a nanocrystalline structure with a smoothersurface [8]. Furthermore, a change of the preferred orientation of the

crystals in the growing film [9], as well as of the phase composition[10,11] was reported. This led for example to an improvement of themechanical [12], optical [13] and electrical [14] properties of thedeposited films. HIPIMS also showed a dramatic improvement whencoating structured [1,15] or three-dimensional substrates [16]. Oneshould also refer to the articles by Helmersson et al. [17], Alami et al.[18] and Sarakinos et al. [19] for additional information on recentdevelopments of the HIPIMS technology.

Furthermore, it was found that when sputtering with the sameaverage power a lower thermal load at the substrate and a lowerdeposition rate compared to DC magnetron sputtering, DCMS, wereachieved [20]. Several explanations and suggestions to overcome thelatter problem have been reported [3,21–26].

Optical emission spectroscopy (OES) is a very versatile tool for theinvestigation of plasma discharges. Simple applications of OES, notrequiring expensive equipment are usually used to identify speciesduring etching or deposition processes. As the OES signal depends onvarious plasma parameters, such as the electron temperature, theelectron density as well as the number density of the particles in theground state, we can gain information on their temporal developmentby observing the plasma-induced emission.

In a recent publication by Hala et al. [27] the plasma-inducedemission of a reactive HIPIMS discharge with a chromium target andargon and nitrogen as working gases was studied. The temporal

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evolution of the intensity of representative lines for each species wasinvestigated using an optical probe and a spectrometer. These resultswere supplemented with time-resolved imaging of the emission ofthe whole discharge. In order to extend this study 2D-imaging wasused employing additional optical bandpass filters which allowed usto directly observe the temporal development of selected speciesthroughout the entire discharge. Furthermore, the cylindrical sym-metry of the magnetron source was exploited to apply Abel inversionfor the calculation of spatially resolved emission profiles. The aim ofthis work is to gain new knowledge on where in the dischargeexcitation and ionisation processes of both the sputtering gas as wellas the sputtered material take place.

2. Experimental

A planar, unbalanced magnetron, cylindrical in shape, V-TechTM

150 supplied by GENCOA Ltd. was installed in the centre of a vacuumchamber with a diameter of 400 mm and a length of 600 mm, whichwas pumped down to a base pressure of 2×10−4 Pa using a turbo-molecular pump backed with a rotary pump. The targets, which were150 mm in diameter and 6 mm thick, were made of titanium (purity99.99%) and aluminium-doped zinc (Zn–5 at.% Al). All experimentswere carried out with argon (purity 99.99%) as the working gas whichwas introduced with a flow rate of 30 sccm. Using a throttle valve thepumping speed could be altered to set pressures of 0.53 Pa and 1.33 Pawith titanium target and 1.33 Pa when the Zn–5 at.% Al–target wasinstalled. A Sinex 3.0 HIPIMS power supply (Chemfilt Ion Sputtering)was used to ignite the discharge. The pulse ‘on’-time was set to be100 μs for all experiments, whereas the repetition frequency waslowered from 100 Hz for sputtering titanium to 50 Hz for sputteringaluminium-doped zinc in order to reduce the thermal load of thetarget. Supplying average powers of 650 W (titanium) and 400 W(Zn–5 at.% Al) to the discharge resulted in peak power densities of1.2–1.3 kW cm−2 and 1.0 kW cm−2 and current densities up to 2.6–3.0 A cm−2 and 1.2 A cm−2 for the titanium and the Zn–5 at.% Al–target, respectively. To monitor the current and voltage waveforms aTektronix TDS3014 oscilloscope, a 100:1 voltage probe P5100 and acurrent probe TCP202 in combination with a 20:1 high currenttransformer CT4 (all supplied by Tektronix) were used.

A schematic diagram as well as a more detailed description of theoptical setup is given elsewhere [28]. In contrast to the experimentsdescribed in Bradley et al. [28], an Andor DH520 iCCD camera with1024×256 pixels and a delay generator DG645 (Stanford ResearchSystems) were used. Various spectral filters were used to observe thelight emitted by selected species as summarised in Table 1. The spatialresolution was calculated to be 0.4 mm and the area of observation,for which results are shown here, ranged from 4.5 mm (blocked by atarget holder) to 50 mm above the target surface. An axial position of0 mm, therefore, equates to the position of the target surface. Lateralpositions from −80 mm to +80 mm were accessible and thedischarge axis is denoted as 0 mm. Hence, the whole width of thedischarge could be observed. A temporal resolution of 1 μs was used. Aprogramme written in-house based on the Abel inversion algorithm

Table 1Summary of the spectral filters used to observe the plasma-induced emission of variousspecies.

Target material Observed species Centre wavelength(nm)

FWHM(nm)

Titanium Ar(I) 752.1 10.6Ar(I) 811.63 9.15Ti(I) 502.55 10.95

Aluminium-doped zinc Ar(I) 811.63 9.15Ar(II) 436.58 4.26Zn(I) 637.92 10.44Zn(II) 591.19 9.53

proposed by Gueron and Deutsch was used for further analysis [29].Assuming the validity of the Corona model, contained in Hutchinson[30], which is a simple electron-impact-radiation model, the mea-sured intensity I depends on the plasma parameters electron densityne, electron temperature Te via the rate coefficient k, as well as thenumber density of particles in the ground state n0 [31]:

I ∝ k Teð Þnen0: ð1Þ

3. Results and discussion

3.1. Temporal development of the plasma-induced emission

The temporal development of the intensity of selected speciesduring the pulse ‘on’-time together with the target voltage anddischarge current waveforms for sputtering titanium (0.53 Pa) andaluminium-doped zinc are displayed in Fig. 1. It is worth noting, thattime-averaged optical emission spectra (not shown here) revealed,that it was not possible to record the emission of argon and titaniumions separated from other species with the current setup. The pulseinitiation is characterised by a rapid drop of the target voltage with atransient of 1.8 kV μs−1 to its maximum magnitude of −670 V and−970 V for the titanium and Zn–5 at.% Al–target, respectively.Afterwards, the target voltage decreases continuously in magnitude

Fig. 1. Temporal development of the target voltage and discharge current waveform incomparison to the intensity of the plasma-induced emission 5 mm above the target:A) Sputtering titanium: Pave=650 W, f=100 Hz, ton=100 μs, and p=0.53 Pa. No datafor argon and titanium ions was recorded, because no spectral range was accessible toobserve these species separately. B) Sputtering aluminium-doped zinc: Pave=400 W,f=50 Hz, ton=100 μs, and p=1.33 Pa. The labels A, B and C are discussed in the text.

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until the pulse terminates. With a delay of about 10 μs the dischargecurrent starts to increase significantly peaking at 38 μs and 50 μs whensputtering titanium and aluminium-doped zinc, respectively. Thedischarge current thendecreases until the pulse terminates after about102 μs, leading to a target voltage and a discharge current equal to zerountil the ignition of the next pulse. Three temporal positions werechosen and marked in Fig. 1A to represent the discharge's develop-ment when sputtering titanium. The first point ‘A’ 10 μs after pulseinitiation was chosen as a representative for the initial stage of thedischarge, while ‘B’ marks the discharge peak current in bothdischarges and ‘C’ describes the position of an additional maximumonly observed in the argonneutral emissionwhen sputtering titanium.

The first emission to peak in both discharges is from argon neutrals.Looking at the results obtained for the titanium target, no significantdifference in the temporal development of the spectral lines 750.4 nm,751.5 nm and 810.4 nm, 811.5 nm can be seen (compare Fig. 1A)which held also true for their spatial distribution (not shown here).The emission of titanium neutrals starts with a short delay and revealsits maximum shortly after the discharge current peaked. Its intensitythen decreases quickly to less than 20% of its maximum at the selectedpoint ‘C’. Complementary results obtained for the sputtering of a Zn–5 at.% Al–target show that the rise of the intensity of argon ions andzinc neutrals starts much later than the argon neutral emission butearlier than the zinc ion emission, which peaks at the same time as thedischarge current. It is worth noting, that the emission of argonneutrals and ions dropped significantly before or shortly after thedischarge peak current occurred.

The temporal development of the plasma-induced emission forsputtering chromium with argon as working gas has been studiedintensively [3,27,32]. Hala et al. [27] divided the discharge into fourstages, namely the ignition, the metal-dominated, a transient and agas-dominated phase. The ignition is characterised by an initialincrease followed by a drop of the argon neutral and ion emission,which can be seen in the results presented here, as well as in thestudies cited above. The replacement of the working gas by sputteredmetal atoms and ions in front of the target (often referred to as gasrarefaction [33]) and the associated rise of the metal (Cr, Ti or Zn)neutral and ion emission are also features shared by all thesedischarges. Hala et al. [27] suggested a certain threshold in the targetvoltage to obtain the metal-dominated stage governed by self-sustained sputtering [34], below which a change into the transientregime in which the target is sputtered mainly by working gas ionslike in DCMS takes place. Thus, the presence of the described stages ofthe plasma clearly depends on whether or not the power supply iscapable of delivering and maintaining the threshold voltage to thecathode. The studies on sputtering chromium [3,27,32] clearlyshowed the presence of the metal-dominated regime due to a hightarget voltage and, depending on the development of the targetvoltage a change into the transient or even the gas-dominated regime(only for [27,32]). In this study, however, a larger target diameter of150 mm was used which facilitates high discharge currents causing arapid drop of the target voltage. At the same time a lower peak targetvoltage had to be chosen in order to maintain a moderate averagepower not to overheat the target. It is therefore doubtful, if thetransition into the pure metal-dominated regime took place.Nevertheless, the temporal development of the intensities (seeFig. 1) and the time-averaged spectra (not shown here) clearlyindicate the presence of a metal-ion-rich plasma when sputteringwith a high instantaneous discharge power. During the second half ofthe pulse the drop of the current density and of the intensities ofmetalneutral and metal ion emission confirms the presence of the transientregime.

Themain difference in the argon neutral emission between the twodischarges is the appearance of a second maximum at around 70 μswith a lower intensity than the first peak in the titanium sputteringplasma. Measurements of the electron density when sputtering a

titanium target under similar conditions as investigated here revealedtwo distinct maxima of the electron density exactly at the sametemporal positions as the two maxima in the argon neutral emissionhere [35]. It was suggested that the first peak corresponds to the peakcurrent and mainly represents the presence of argon ions. Further-more, the second maximum was attributed to the ionised, sputteredtitanium flux arriving at the probe position. While the first maximumis in reasonably good agreement with the previous Langmuir probestudies, the investigations performed here revealed that the secondmaximum in the argon neutral emission occurred at the same timeindependant from the position in the discharge which disagrees withthe explanation given in that paper. An increase of the electron densitydue to ionisation of sputtered titanium atoms is most likely becausetheir intensity shows a dramatic reduction between its peak att=40 μs and later at t=70 μs, but no transit of this phenomenon couldbe observed as one would expect from the travelling titanium atomflux.

3.2. Titanium target sputtering

The spatially resolved intensity distributions for sputtering atitanium target at an argon pressure of 0.53 Pa are shown in Fig. 2. Fora better comparison with the temporal development of the spatiallyresolved emission of the plasma, images of a discharge running in DCmode (p=0.53 Pa, P=500 W) mode were recorded. Both imagesreveal a well-pronounced torus structure as expected by the magneticfield configuration. The overall maximum appears close to the target,where the maximum electron density is expected due to the electronconfinement by the combination of the magnetic and electric field.Moving away from the target the intensity decreases and themaximum is situated closer to the centre of the discharge, determinedby the position of the magnetic field lines oriented parallel to thetarget surface.

Starting with the emission of argon neutrals in the HIPIMSdischarge, the first image, position A, reveals a toroidal shell, similarto the DC case, except for one additional feature: the overallmaximum of the emission can be found in a region detached fromthe target surface and travelling away with a speed of (1.71±0.25)km s−1 when comparing images in the interval of 5 μs to 20 μs. Thismoving maximum of intensity is consistent with an ion acoustic waveoriginating from the rapid sheath motion during the steep targetvoltage transient and has already been observed in pulsed-DC, as wellas in HIPIMS discharges [27,28,36]. Progressing further in the pulse tothe discharge peak current the emission of the neutral sputtering gasreveals an intensity distribution which differs substantially from theDC discharge. The maximum of the intensity is emitted by a region inthe centre of the discharge whereas a shell can hardly be seen. Inaddition, the area immediately above the racetrack shows a decreasedemission. Spatially resolved electron density measurements per-formed by Bohlmark et al. [5] show, that the electron density does notdecrease in the area of observation during this particular time. Nodrop in the electron temperature has been reported, either [35]. Thelast plasma parameter in Eq. (1) influencing the emission is thenumber density of particles in the ground state. This can either becaused by a high degree of ionisation of the sputtering gas or by adecrease of the argon density due to gas rarefaction. Ionisation ofargon can be excluded because of the temporal development of theargon ion signal observed when sputtering the Zn–5 at.% Al–targetwhich clearly shows a decrease of the emission even before thedischarge current peaks. In addition, simulation results reported byKadlec [37] showed that rarefaction of the working gas was mostpronounced in the plasma torus above the racetrack, where the mostsignificant lack of argon neutral intensity occurred. This supports thesupposition that the distorted intensity distribution obtained for point‘B’ can be attributed to the rarefaction of argon. Finally, an intensityprofile similar to the one recorded for the DC discharge was obtained

Fig. 2. Spatial distributions of the emission of argon and titanium neutrals when sputtering titanium. The images were obtained after analysing the measured data with Abelinversion. Spectral filters with the centre wavelengths 750 nm and 500 nm were used for Ar(I) and Ti(I), respectively. All images have been recorded with a pressure of 0.53 Pa andaverage powers of 500 W and 650 W for DCMS and HIPIMS, respectively. Three temporal positions are selected: the initial stage ‘A’ is taken at 10 μs, ‘B’ represents the discharge peakcurrent at 40 μs and ‘C’ refers to the additional maximum in the argon neutral emission at 70 μs. Each image is normalised to itsmaximum intensity. The temporal development of theintensity can be seen in Fig. 1A.

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at point ‘C’, as supposed by Hala et al. [27]. It is worth noting that theintensity maximum in the torus is invariant at distances between5 mm and 40 mm from the target. From our results, we cannot deducewhether a density gradient in the working gas is still present. Inaddition, the underlying mechanism for the creation of the secondpeak in the electron density is not fully understood, yet. Therefore,this phenomenon will be the focus of further studies.

The emission of titanium neutrals shows a well-defined torus-likestructure 10 μs after pulse initiation. The radial position of themaximum is the same as compared to the image taken at the DCdischarge. A significant difference can be seen from the extent of theemission. Titanium neutrals in the DC discharge emit intensity as highas 20% of the overall maximum even 50 mm away from the targetsurface,whereas such ahigh intensity is limited to an area up to 10 mmin front of the target. This is probably caused by the runtime effect ofthe on-setting sputtering process. Atoms sputtered at a time as early asthe pulse initiation would need a velocity of 1 km s−1 to reach aposition 10 mm above the target surface. This is in the same order ofmagnitude as the drift speed of 2.1 km s−1 reported by Macák et al.[38]. Although the pressure in their study was higher (1.1 Pa), one hasto consider that sputtering probably occurs with a delay of a fewmicroseconds in respect to the initial target voltage drop. Furthermore,the electron density is also expected to decrease when increasing thedistance to the target, hence the measured intensity is decreasedfurther. Both effects are responsible for the calculation of an under-estimated velocity, thus our imaging results are in reasonably goodagreement with the onset of the sputtering process. As the dischargeprogresses in time, one would expect the titanium neutral emission toextend further into the plasma bulk, but this can hardly be seen in theimage taken shortly after the discharge current peak at 40 μs. Bycontrast, the emission seems to split up into two distinct peaks with alack of intensity at the radial position fromwhere the highest intensity

is expected to be emitted. From the same considerations as for argonneutrals, the electron density and temperature can be ruled out as anexplanation [5,37], thus the number density of titanium atoms has tobe taken into account. A preferred sputtering at the position directlybelow the intensitymaxima can be excluded because thiswould implythe existence of two distinct racetracks which could not be confirmedby any of our targets being run in a HIPIMS discharge and it is also notreasonable from the magnetic field configuration. If we assume mostsputtering taking place at a radial position between45 mmand50 mmwhere the racetrack ismost pronounced [39], most titanium atoms areejected at this position, but the neutral density is already lost whenlooking at the spatially resolved emission. It is therefore suggested thatthe sputtered titanium atoms are already ionised by the extraordi-narily high electron density in front of the target before entering thearea of observation, hence the number density of titanium atoms andtheir intensity are decreased. When looking at the temporal position‘C’, the splitting is reduced and the emission is more extended into theplasma bulk. Thus, it can be assumed that less ionisation of thesputtered titanium occurred at this time. The further developmentreveals that bothmaximamerge again later during the pulse ‘on’-timeand the intensity distribution resembles the DC case.

3.3. Aluminium-doped zinc target sputtering

To complement and validate the findings for the titaniumsputtering, the magnetron source was equipped with a Zn–5 at.%Al–target to extend the investigation to argon and metal, in this casezinc ions. Fig. 3 shows the intensity distribution of argon neutrals, ionsas well as zinc neutrals and ions during the discharge peak current,temporal position ‘B’. The emission of argon and zinc neutralsconfirms the findings, such as rarefaction of argon and the splittingof the metal neutral emission, as obtained for sputtering titanium. In

Fig. 3. Spatial distributions of the emission of argon and zinc neutrals and ions when sputtering an aluminium-doped zinc target. The images were obtained after analysing themeasured data with Abel inversion. All images were recorded at the same temporal position ‘B’ 50 μs after pulse initiation when the discharge current peaks. The target was sputteredusing an argon pressure of 1.33 Pa and an average power of 400 W.

Fig. 4. Comparison of the target current density FWHM modelled according to Wendtet al. [41], the splitting of the zinc neutral emission and the width of the argon ionemission in dependence on the instantaneous discharge power. Closed symbols indicatethe increase of the discharge power, while open symbols refer to the decreasingdischarge power.

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contrast to the two distinct maxima in the zinc neutral emission theintensity distribution of zinc ions only shows one single maximumclose to the target at the expected radial position between 45 mm and50 mm. This supports the supposition that sputtered metal atoms areindeed ionised by the high electron density above the racetrack, whileless ionisation takes place close to the discharge axis and the outerregion where the electron density is lower. Finally, the image takenfrom the argon ion emission at the discharge peak current reveals awell-established plasma torus. The most-pronounced feature of theargon ion emission is the increased width of the maximum close tothe target compared to the argon neutral emission in the DCdischarge. This fits well to the wide emission found for zinc andtitanium neutrals, because the source of these species is sputtering byargon and metal ions. In addition, investigations performed by Clarkeet al. [39] also revealed an increased width of the current densitydistribution at the target surface in HIPIMS, which is at least partlycarried by impinging argon ions. It can be explained by the decrease ofthe electron confinement due to a lowered magnetic field strengthcaused by the high azimuthal currents present close to the targetsurface [40]. According to the model developed by Wendt andLieberman [41], the width of the target current density FWHM incentimetres is given:

FWHM = 94:7V1=10I1=5

B4=5 ; ð2Þ

where V is the target voltage in volts, I the discharge current in ampereand B is the magnetic field measured in Gauss. While target voltageand discharge current were recorded during operation, the staticmagnetic field strength of 40 mT (400 Gs) was measured without anydischarge. Fig. 4 shows the calculated width of the target currentdensity in comparison to the width of the argon ions and the splittingof the zinc neutral emission as a function of the power delivered to theplasma. It can be seen that both values increase with increasingdischarge power and the maximum is reached when the dischargepower peaks. In addition, both widths also decrease in the same waywhen the discharge power decreases in the second half of the pulse.This agreement is reasonable as argon ions created in front of thetarget are drawn towards the cathode and accelerated perpendicu-larly to it by the electric field. Impinging ions and their emittedsecondary electrons contribute to the target current. The generallylower width measured for the argon ion emission in comparison tothe width of the target current density distribution (see Fig. 4) mightbe explained by the fact, that axial distances closer than 4.5 mm to thetarget surface were not accessible with the current setup due to thetarget holder. In addition, the assumption that the intensity onlydepends on the number density of argon ions in the ground state

falsifies the result, as the electron density should reveal a similarbehaviour, thus reducing thewidth of the emission further. Hence, thewidth of the argon ion emission is thought to underestimate the argonion density.

The distance between both intensity maxima of Zn(I) emissionshows the same trend as the argon ion emission. This behaviour isexpected as the argon ions do not only contribute to the target currentdensity but also cause sputtering of the target material. Thus, theFWHM of the sputtered zinc density increases in the same way aspredicted byWendt's model. At the same time, the width of the metalion density in front of the target is expected to increase in the samemanner as the width of the argon ion density does. This could not beobserved for the emission of Zn(II), whosewidth remained constant at(20±1)mm while ramping up the discharge power and increasedwhen the discharge power decreased. Taking into account thetemporal development of the zinc ion emission, which revealed apronounced maximum at the discharge peak current one can deducethat the contribution of the zinc ion density is highest during this time,thus leading to the maximum splitting of the zinc neutral emission.

4. Conclusion

Spatially and temporally resolved 2D-imaging revealed substantialdifferences between the development of the emission of argon andmetal neutrals and ions. The pulse initiation is accompanied by a

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maximum in the emission travelling away from the target which isconsistent with an ion acoustic wave originating from the rapidlyexpanding sheath during the voltage transient. The emission of argonundergoes a significant change during the pulse and gas rarefaction infront of the target could be confirmed by the temporal developmentas well as the spatial distribution of the emission. The spatialdistribution of the argon ion emission showed an increase of itsmaximum in front of the target, which could be attributed to adecreased magnetic confinement of the electrons. This led to a widerdistribution of the argon ions in front of the target which could belinked to target current density measurements and its modelling. Thewider profile of the argon ion density caused an increased area oftarget sputtering which could be confirmed by the metal neutralemission of both target materials. In addition, the emission of metalneutrals manifests two well-distinct maxima in direct vicinity to thetarget. It has been demonstrated that the lack of intensity in betweenthese maxima is caused by ionisation of the sputtered metal fluximmediately above the target surface.

Acknowledgements

The authors would like to thank Dr. P. Bryant for his help innumerical computation, as well as Prof. F. Richter from ChemnitzUniversity of Technology for lending the delay generator. We wish tothank Mr. A. Roby, too, for his technical support. The work wasfinancially supported by the EPSRC under reference EP/E003397/1.

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